Every year, many millions of tons of used tires accrue globally, of which, about 25% are utility vehicle tires. In Europe alone, about 3.2 million tons of used tires accrue per year, the proportion of used utility vehicle tires being about 0.8 million. A large portion of the used tires is utilized for energy purposes in the cement industry, and is thus no longer available for a further physical utilization, in the face of increasing raw material scarcity.
To date, there are only very limited means of physical utilization for used tire material in the form of granules or powder in mixtures for new tires. The most important physical utilization is the use of granulated rubber or powdered rubber produced from used tires, also called ground rubber, of various particle sizes in rubber mats or other dynamically unstressed rubber products.
In the production of granulated rubber or powdered rubber from utility vehicle tires, there are basically two different products:
The first product is the granulated/ground whole tire material. Since the whole tire is granulated/ground, it contains all the tire mixtures in a tire from the inner airtight butyl- or halobutyl-containing inner liner mixture up to the tread made from natural rubber. In order to keep the air pressure in the tire constant over a long period, all tires have an inner liner mixture comprising predominantly butyl rubber, chloro rubber or bromobutyl rubber as the rubber. If the whole tire is comminuted and ground, the granules or powder always also contain the inner liner mixture. Large amounts of this material are supplied on the market, and some is used as a filler in asphalt.
A further type of granulated/ground tire material which is produced to a very limited degree consists of shaved-off treads and sidewalls of reconditioned utility vehicle tires. This material consists almost exclusively of natural rubber.
The known granulated rubbers or powdered rubbers from utility vehicle tires can be added as an aggregate in small amounts to freshly produced tire mixtures in tire production. However, the use thereof is very limited in terms of amount, since the granulated tire particles act like small foreign bodies in the fresh tire mixture. Among other causes for this behavior, one is that the strength and extension of the granulated used tires differ significantly from those of the fresh mixture after vulcanization in the tire mixture. Vulcanization then vulcanizes the granulated tire particles for a second time in the new product. The first vulcanization of the granulated tire material was already conducted in the used tire, the starting material for the granulated tire material. The used tire particles which have now been vulcanized for the second time show a difference in the strength/extension characteristics at the interface to the fresh tire mixture, which can adversely affect dynamic performance. In many rubber products which are not subject to dynamic stress, for example rubber mats, stands for mobile traffic signs for roadworks sites or inclined sidewalk ramps, it is possible to add granulated tire material or else granules of other rubber products to the fresh mixtures because there is no dynamic stress as in tires in this case.
In order nevertheless to enable high-value utilization of granulated used tires for use in tires, it is known and customary to regenerate this granulated material. In a regeneration process, the vulcanized granulated tire material is plasticized, meaning that it is converted from the elastic state to a plastic state, by degrading the rubber chains and the sulfur bridges formed by the vulcanization.
Numerous processes for regeneration or devulcanization of vulcanized granulated/ground tire material are being employed. All processes have the aim of regenerating, that is, splitting up, the crosslinking structure. The effect of thermal and mechanical energy in roll mills or in mixers, with the aid of different polymer chain-degrading and sulfur bridge-degrading chemicals, depolymerizes the molecules of the vulcanized granulated/ground used tire material, and also degrades sulfur bridges. This operation is generally referred to as regeneration.
The now plastic product which arises in the processes and has a defined viscosity is available on the market as a regenerated material for various additions to tire mixtures and industrial rubber mixtures, and shows the adverse properties of unregenerated granulated/ground tire material in dynamically stressed products to a distinctly reduced degree.
However, the butyl, chloro or bromobutyl rubber components present still have an adverse effect in the granulated or powdered material.
Because of their molecular structure, these granules of butyl, chloro or bromobutyl rubber show incompatibility when mixed/blended with natural rubber and other rubber types in freshly produced mixtures of natural and synthetic rubbers. They form domains of undistributed butyl and halobutyl rubber. As shown by studies and experience from the tire industry, these rubber types act like a separation layer in the mixture, which can lead to distinct shortcomings in performance in the product. These rubbers are very difficult to homogenise in mixtures with other rubbers, for example natural rubber, and their chemical constitution leads to poor binding in fresh mixtures.
This adverse behavior of the mixtures with butyl and halobutyl rubber, for example the inner liner mixtures, is well known in the tire industry and limits the use of regenerated material from granulated rubber or powdered rubber from whole utility vehicle tires as a valuable mixture addition to a considerable degree. A considerably greater use of natural rubber-containing regenerated material from granulated/ground whole utility vehicle tire material would therefore be possible if the proportions of the butyl, chloro or bromobutyl rubbers from the material could be reduced prior to the regeneration process.
Also in the case of granulated rubbers and powdered rubbers which are produced from used rubber products other than tires, there may be incompatibilities in reutilization in the case that several rubber types are present in the rubber product.